Is it correct that this sentence is supposed to have a relative inside?

I am reading a book and there’s one sentence: "race-mixing becomes a crime worse than treason." (from Born A Crime by Trevor Noah)

I wonder how "crime" and "worse" connect? is it okay to just put the adjective behind or there is " which is" between them?

So the original should be " a crime (which is) worse than treason"?

Thanks!!!